1. Field of the Invention
The present invention generally relates to image forming apparatuses such as laser printers, laser facsimiles, and digital copying machines, optical scan devices used for such image forming apparatuses, and image forming methods, and more particularly, to an image forming apparatus and image forming method that perform correction of a shift (displacement) of a scan position of laser scan, and pixel position control of optical scan devices.
2. Description of the Related Art
A color laser printer is equipped with a laser scan optical device having laser scan optical units corresponding to four colors (Y: yellow, M: magenta, C: cyan, and BK: black). Each of the laser scan optical units includes a laser diode emitting a laser light beam, a polygon mirror deflecting the laser beam, and the like.
The laser beam emitted from-the laser diode is deflected by the polygon mirror after modulation based on image data, and scanned on a photo conductor line by line. This is called main scan, and a 2-dimensional image is formed on the photo conductor by the main scan and sub scan by rotation of the photo conductor. Such a color laser printer has advantages in that modulation for each dot is easy, there is little unevenness of an image in the main scan direction, and the like.
By the way, each of the laser scan optical units corresponding to each of the colors has a different linearity property in the main scan direction according to variations in lens performance and in mounting of the lens. In addition, the linearity property is varied by elongation of a component according to temperature changes. Such variations in the linearity property cause variations in registration.
(Prior Art 1)
For this reason, in order to correct the variation in registration, a conventional color laser printer prints a pattern for detecting the variation in registration, reads the pattern by a CCD or the like, and detects the amount of variation in registration. Then, adjustment of registration is performed by modulating a dot clock and correcting the variation in registration so that shift (displacement) of the print position in the main scan direction is eliminated.
In addition, in a case of a tandem laser scan optical device, color matching of respective laser scan optical units is performed by standardizing the temperature characteristic of each of the laser scan optical units.
However, in the above-described conventional method of adjusting the registration, errors in reading the pattern for detecting the variation in registration are considerable. Thus, there is a problem in that the registration correction cannot be correctly performed, and thus it is impossible to obtain a high quality image. Additionally, since a CCD or the like for reading is required, there is also a problem in that the size of each of the laser scan optical units is increased and cost increases.
Further, in the method of standardizing the temperature characteristic of each of the laser scan optical units so as to perform the color matching of each of the laser scan optical units of the laser scan optical device, it is difficult to standardize the temperature characteristics due to limitations of accuracy of components. That is, the scan position of a laser beam on the photo conductor is shifted according to the temperature changes for each of the laser scan optical units.
A problem will not be caused if the scan position of the laser beam shifted by the temperature changes is the same in each of the laser scan optical units. However, the scan position of the laser beam of each of the laser scan optical units varies according to the characteristic of each of the laser scan optical units and the difference of temperatures between the laser scan optical units. This causes color shift. Especially, the shift of scan position introduced by the difference between the thermal expansion of the lens and that of the chassis or the like has a great influence.
(Prior Art 2)
In Japanese Laid-Open Patent Application No. 2-50176, recording shift is corrected by generating a pattern designation signal corresponding to a detected temperature, generating a line designation signal with a pattern designated by the pattern designation signal during 1-line main scan, and scanning according to image data of a line designated by the line designation signal.
However, in such a correction method, though it is possible to correct the shift in the sub scan direction introduced by the temperature change, there is a problem in that it is impossible to perform correction of shift of registration in the main scan direction.
(Prior Art 3)
Considering such a condition, in the invention in Japanese Laid-Open Patent Application No. 2001-201702, correction of print position is performed such that, in a tandem laser scan optical device, a laser scan optical unit having a temperature sensor is provided for each of the colors Y, M, C and K, the linearity characteristic and data of temperature gradient in the main scan direction that are measured for each of the laser scan optical units are maintained, an amount of correction of print position difference according to an error in mounting fθ lens and temperature changes is calculated, and based on the calculated amount, the modulation of a dot clock and correction of the print position difference are performed.
However, in the prior art 3, the cost is high since the tandem laser scan optical device includes four sets of a light source, a polygon motor, a fθ lens, and a temperature sensor for four colors, that is, Y, M, C and K.
In addition, the tandem laser scan optical device further includes four polygon mirror motors that are heat sources therein. Thus, the tandem laser scan optical device increases the temperature by self-generating heat, and performs correction by detecting the temperature for each of the laser scan optical units. Accordingly, energy saving is not achieved.
Further, correction control is complex since the tandem laser scan optical device uses as many as four temperature sensors.
Concerning such aspects, as mentioned above, the print position difference occurs according to the characteristic of each of the laser scan optical units and the difference of temperature between the laser scan optical units. However, by arranging a plurality of scan optical systems close together, it does not matter if the temperature changes of the scan optical systems are regarded as the same. Thus, it is possible to manage with a smaller number of temperature sensors than the number of the laser scan optical units. Hence, the correction control can be simplified.
Moreover, in the prior art 3, a modulation means of the dot clock for performing printing correction is not specified. Thus, the prior art 3 lacks specifics for realization.
By the way, an image forming apparatus such as a laser printer, a laser facsimile, and a digital copying machine includes an optical writing device or an optical scan device having an optical deflector and a surface to be scanned. The optical deflector deflects (reflects) a beam from a light source. Information is written on the surface to be scanned by scanning the surface with the beam deflected by the optical deflector. Generally, a laser light source such as a semiconductor laser is used for the light source. Hereinafter, the optical scan device is referred to as a “laser scan device”, and an optical system thereof is referred to as a “laser scan optical system”.
As a factor that degrades writing quality or quality of forming images, there is variation in scan speed on the surface to be scanned, which surface is constituted by a surface of a photo conductor. When there is variation in the scan speed, as a result, shifts of dot positions (hereinafter referred to as “dot position shifts”) occur. Especially, in color images, the variation in the scan speed causes color shift, resulting in degradation in color reproduction characteristics and resolution. Thus, in order to obtain high-quality images, one condition to be met is that the variation in the scan speed be eliminated. However, in prior art, there is a limit to eliminating the variation in the scan speed. The factors that cause the variation in the scan speed in the laser scan optical systems are as follows.
1. fθ characteristic of a scan lens is not sufficiently corrected
2. degradation in precision of optical components of the laser scan optical systems and accuracy in mounting to a housing
3. fθ characteristic is degraded due to variation in focal distance, which variation is caused by deformation of the optical components and variation in refraction factors of the optical components, according to variation in the environment such as temperature and moisture in an image forming apparatus
In particular, it is possible to avoid the dot position shift in the main scan direction caused by the environmental variation (#3 of above factors), even if optical tuning or electrical correction is performed on the image forming apparatus before shipping. In order to meet the needs for higher quality these days, it is necessary to solve this problem. More specifically, in color image forming apparatuses such as color laser printers that scan with a light beam for each of a plurality of colors, the dot position shift in the main scan direction is a great factor for degrading image quality. Thus, it is important to eliminate the dot position shift so as to maintain image quality.
Prior art for correcting the dot position shift in the main scan direction are as follows. As an example of including a means for correcting a start position for writing of an image forming position of each color with an error within 1 clock, there is an invention described in Japanese Laid-Open Patent Application No. 2000-238319. As an example of adjusting the start position and end position for writing in the main scan direction, there is an invention described in Japanese Laid-Open Patent Application No 2000-289251.
However, in neither of the inventions described in the above-described applications, is it possible to correct the influence of the dot position shift in the main scan direction caused by optical systems and deflectors.